Engine balancing devices are known. For instance, Japanese UM publication No. 5-39233 discloses a balancing device in which a pair of balance shafts each provided with a counter weight for canceling the unbalance force of the second order produced by pistons of an engine are disposed under the crankshaft in the oil pan, and the rotation of the crankshaft is transmitted to the balance shafts via a chain/sprocket mechanism or a gear mechanism. A similar balancing device is disclosed in U.S. Pat. Nos. 4,703,724 issued Nov. 3, 1987 to C. Candea et al. and 4,703,725 issued Nov. 3, 1987 to W. L. Weertman.
In such a balancing device, because the vibration control diminishes in effectiveness if the balance shafts deflect, the journal portion for rotatably supporting each balance shaft is desired to have as high a rigidity as possible. Therefore, the journal portion of a balance shaft generally consists of a solid structure having a fully circular cross section.
However, it is desired to minimize the overall weight of the balance shaft because of the increasing demand for the weight reduction of engines. Also, the balance shaft is required to have a certain unbalance mass. Therefore, the mass of the balance shaft is desired to be allocated to the unbalance mass as much as possible, and the mass which does not contribute to the generation of unbalance for canceling that of the engine, such as the mass found in the journal portion is desired to be minimized.
Such an effort to reduce the mass or weight of the various parts of a balance shaft should be implemented without compromising the required properties of the balance shaft such as the adequate bending rigidity and the load bearing capability of the journal.
Also, because the balance shaft is required to be installed in a relatively limited recess within the confine of an engine, it is important that the assembly of the various components of the balancing device can be executed in a simple manner.